The opening chapters of Operating Systems look easy until an exam turns them into precise questions: what actually counts as multiprogramming, which mode a signal handler runs in, what does and does not interrupt a running process, and what the operating system must save on a context switch. Each of the 12 below comes from a real paper: GATE 1998 through GATE 2015, plus UGC NET 2020 and a 2025 state recruitment paper. Attempt every question before you read the answer, then follow its link for a fuller walkthrough inside the introduction to OS and process management modules.
What an operating system is and does
Q1. Which of the following is the primary purpose of an operating system? (TPSC 2025, see the OS basics module)
(a) Manage hardware resources
(b) Provide network security
(c) Execute applications
(d) Manage user accounts
Answer: (a) Manage hardware resources.
The operating system's core job is to manage the CPU, memory, storage and devices and to present a clean platform on which applications run. Executing applications and handling accounts are things it enables, but resource management is the defining purpose.
Q2. Which of the following is NOT a system software? (UGC NET 2020, see the OS basics module)
(a) Compiler
(b) Operating System
(c) Application Software
(d) Editor
Answer: (c) Application Software.
System software runs and supports the machine itself: the operating system manages hardware, while compilers and editors support program development. Application software is written for user tasks like word processing or browsing, so it sits in the opposite category.
Q3. Which combination of features characterises a multiprogrammed OS? (a) more than one program may be loaded into main memory at once; (b) if a program waits for an event such as I/O, another is scheduled; (c) if a program terminates, another is scheduled. (GATE 2002, see the full solution)
(a) a only
(b) a and b
(c) a and c
(d) a, b and c
Answer: (b) a and b.
Multiprogramming means keeping several programs in memory and switching the CPU to another whenever the current one blocks on I/O, so features (a) and (b) together define it. Feature (c), moving on after a program terminates, also happens in plain batch processing, so it does not distinguish multiprogramming.
Q4. Which of the following is an example of a spooled device? (GATE 1998, see the full solution)
(a) The terminal used to enter input data for a program
(b) An output device used to print the output of a number of jobs
(c) The secondary memory device in a virtual storage system
(d) The swapping area on a disk used by the swapper
Answer: (b) A printer serving many jobs.
Spooling queues work for a slow device so faster processes need not wait on it, and the printer is the textbook case: jobs pile up in a spool and print in turn. A terminal is interactive, and swap and virtual-storage areas are memory-management mechanisms, not spooling.
System calls, dual mode and OS abstractions
Q5. To change the CPU mode from privileged to non-privileged, what is needed? (GATE 2001, see the full solution)
(a) A hardware interrupt
(b) A software interrupt
(c) A privileged instruction that does not generate an interrupt
(d) A non-privileged instruction that does not generate an interrupt
Answer: (c) A privileged instruction.
Going the other way, non-privileged to privileged, must be a controlled trap, but here the CPU is already in privileged mode and simply lowers itself. It does that by executing a privileged instruction that clears the mode bit, no interrupt required.
Q6. A processor needs a software interrupt to (GATE 2001, see the full solution)
(a) test the interrupt system of the processor
(b) implement co-routines
(c) obtain system services that need execution of privileged instructions
(d) return from a subroutine
Answer: (c) obtain system services.
A software interrupt, or trap, is how a user program crosses into the kernel to request a protected service such as I/O. The CPU switches to privileged mode and jumps to the handler, which is exactly the system-call mechanism.
Q7. A Unix user-level process traps Ctrl-C and runs a signal handler that saves files before terminating. In which mode does the signal handler execute? (GATE 2005, see the full solution)
(a) kernel mode
(b) superuser mode
(c) privileged mode
(d) user mode
Answer: (d) user mode.
The kernel does the delivery work in kernel mode, arranging the stack and registers, but the handler itself is user code registered by the process. It therefore runs in user mode, like any other part of the application.
Q8. Match each OS abstraction to what it abstracts: A. Thread, B. Virtual address space, C. File system, D. Signal, against 1. Interrupt, 2. Memory, 3. CPU, 4. Disk. (GATE 1999, see the full solution)
(a) A-2, B-4, C-3, D-1
(b) A-1, B-2, C-3, D-4
(c) A-3, B-2, C-4, D-1
(d) A-4, B-1, C-2, D-3
Answer: (c) A-3, B-2, C-4, D-1.
A thread abstracts a CPU's execution stream, a virtual address space abstracts physical memory, a file system abstracts disk storage, and a signal is an interrupt-like notification delivered to a process. Reading each abstraction as "what hardware does this hide" fixes all four pairs at once.
Process states and context switching
Q9. Which of the following does not interrupt a running process? (GATE 2001, see the full solution)
(a) A device
(b) Timer
(c) Scheduler process
(d) Power failure
Answer: (c) Scheduler process.
Devices raise hardware interrupts, the timer fires periodic interrupts for time-slicing, and a power failure triggers a critical interrupt. The scheduler is not an external event; it is the kernel code that runs after an interrupt has already occurred, so it does not itself interrupt anything.
Q10. The maximum number of processes that can be in the Ready state on a system with n CPUs is (GATE 2015, see the full solution)
(a) n
(b) n squared
(c) 2 to the power n
(d) Independent of n
Answer: (d) Independent of n.
At most n processes can be Running at once, but the Ready queue holds every process that is runnable and simply not yet scheduled. That count is bounded by the total number of processes, not by how many CPUs exist, so it is independent of n.
Q11. Which action is typically NOT performed when switching context from process A to process B? (GATE 1999, see the full solution)
(a) Saving A's registers and restoring B's saved registers
(b) Changing address translation tables
(c) Swapping out the memory image of A to disk
(d) Invalidating the translation look-aside buffer
Answer: (c) Swapping out A's memory image.
A context switch saves and restores CPU state, switches the page-table base, and flushes stale TLB entries. Writing A's entire memory image to disk is a separate swapping decision made under memory pressure, not a routine part of every switch.
Q12. Which of the following need not necessarily be saved on a context switch between processes? (GATE 2000, see the full solution)
(a) General purpose registers
(b) Translation lookaside buffer (TLB)
(c) Program counter
(d) All of the above
Answer: (b) The TLB.
Registers and the program counter are process state and must be saved to resume correctly. The TLB is a hardware cache of translations that is simply flushed or tagged, not saved and restored, because it can always be rebuilt from the page tables.
How OS basics are tested in GATE and NET
These twelve turn on the exact boundary of a definition rather than on a calculation. Multiprogramming needs both memory residency and a switch when a program blocks on I/O; the TLB is flushed rather than saved; the Ready queue is bounded by the number of processes, not the number of CPUs. In the exam a nearly-right answer scores nothing, so revise by stating the precise term instead of the general idea, "flushed, not saved" for the TLB and "manages hardware resources" for what an operating system is for.
Three ideas carry the rest of the OS syllabus: what an OS is, with the multiprogramming and spooling facilities it provides (Q1-Q4); the dual-mode boundary that every system call crosses (Q5-Q8); and process states with the real cost of a context switch (Q9-Q12). Getting those three precise makes scheduling, synchronisation and memory management far easier to reason about later.
For the theory behind every answer, work through the introduction to OS module and the process management module, then see how the whole subject fits together in our Operating Systems for GATE breakdown. GATE aspirants get the full sequence inside GATE Guidance by Sanchit Sir; NET aspirants can start from the NET CS category page. Solve, review your misses, and come back a week later.
